Investigation into the long-finned pilot whale (Globicephala melas) mass stranding event, Tolsta, na h-Eileanan Siar, July 2023

Section 1: Background and context

A mass stranding event (MSE) is described as ‘a stranding event involving two or more cetaceans of the same species, other than a mother and calf, stranding at the same time and place’ (IWC, 2016).

On 16 July 2023 one of the UK’s largest fatal mass stranding for decades, comprising 55 long-finned pilot whales (LFPW) (Globicephala melas) occurred on Traigh Mhòr beach, North Tolsta, Isle of Lewis.

The first report from a member of the public was made, via Police Scotland, at around 07:00 to British Divers Marine Life Rescue (BDMLR). The initial report indicated approximately 55 dead animals, including both adults and calves, however, it was subsequently confirmed that 15 of these animals were still alive.

The live animal stranding response was hampered by Tosta's remote location, creating logistical challenges including poor or absent mobile phone communication, difficult vehicle and personnel access and equipment and transport limitations. Poor weather conditions with heavy rain and strong winds further compromised operations, making human safety the overriding priority throughout the incident and ultimately limiting rescue options for the stranded animals. Only one of the 55 animals was successfully refloated and the remainder either died or were euthanised on welfare grounds.

Long-finned pilot whales are an offshore (pelagic) dolphin species known to form matrilineal pods with strong intergenerational bonds and strong social cohesion. This species is known to mass strand; in some cases, such events can involve many hundreds of animals. This MSE offered an opportunity to explore questions of disease prevalence, feeding ecology and contaminant burdens within a large group of animals with a shared recent history. Such data can help inform an understanding of potential factors leading to the stranding and wider questions of ecology and human impact on this species.

This report presents the pathological findings of the investigation and examines broader ecological factors to identify probable causal drivers of the event. More detailed data on specific ecological and life history aspects will be published in due course through the peer review process.